Inquiry: in the arhat’s past lives the causes and conditions for being subject to embodiment necessarily ought to have been eradicated, [in light of this] they dwell where to perfect buddhahood?
Response: when attaining arhatship, the three realms’ myriad outflows’ causes and conditions are exhausted, there is no more birth again in the three realms. There is a pure buddha land,
beyond the three realms, where not even the word affliction has a name, in this kingdom of the Buddha, they hear the dharma flower sūtra [i.e. the Lotus Sūtra], with this they perfect buddhahood.
As in the dharma flower sūtra’s words: "There are arhantaḥ[,] for example[, who’ve] not heard the dharma flower sūtra,
themselves they call [’]ones who have attained cessation[’]; I in another realm for them speak this matter, you all shall become samyaksaṁbuddhāḥ."
Furthermore, this pure land, what relation does it have to ākaniṣṭaghanavyūha?
from Buddhism https://www.reddit.com/r/Buddhism/comments/9zh4hz/amitābhas_sukhāvatī_the_lotus_vault_world/